Which Saddle?

I've done~600 miles this month on an old Arione, perfectly comfortable.

My lower back, however, is FUBAR.

That could be 20mi per day which is certainly not far enough to tell if a saddle is working or not. Of course you could also have ridden 600mi in one 'sitting' which would mean quite a lot more about how well your arse meshes with an Arione.